Keying drive circuits particularly for electronic musical instruments

ABSTRACT

An apparatus is provided for producing a variable signal which utilizes a transistor, a capacitor connected between the base and the collector electrodes of the transistor, a first resistor connected in series with a switching means which are jointly connected between a first source of control potential and the base electrode, a second source of control potential connected via a second resistor to the base electrode, and means for developing an output signal at the collector electrode. By this circuit, when the switch means is closed, the rate of onset or attack of the output signal is linear, and when the switching means is opened, the rate of decay of the output signal is exponential.

United States Patent [72] Inventor Alfred B. Freeman SkoltieJll. [21] App1.-No. 010,087 [22] Filed Feb. 16, 1970 I Continuation of application Ser. No.

599,935, Dec. 7, 1966, now abandoned. [45] Patented Feb. 16, I971 [73] Assignee Chicago Musical Instrument Co.

Chicago, Ill.

[54] KEYING DRIVE CIRCUITS PARTICULARLY FOR ELECTRONIC MUSICAL INSTRUMENTS 29 Claims, 4 Drawing Figs.

[52] U.S. Cl. 84/l.26; 328/78; 325/182 [51] Int. Cl Gl0h H02 [50] Field ot'Search 84/1.13, 1.26; 328/78; 325/161, 164, 170, 182; 307/293; 332/31, 40

[56] References Cited UNITED STATES PATENTS 2,113,371 4/1938 Dyson 325/170x 3,196,201 7/1965 McDonald 84/126 Sharp 1.

Mullard REFERENCE MANUAL OF TRANSISTOR CIRCUITS August 1961, p. 261 relied upon Primary Examiner-W. E. Ray Assistant Examiner-Stanley J. Witkowski Attorney-Hill, Sherman, Meroni & Simpson ABSTRACT: An apparatus is provided for producing a variable signal which utilizes a transistor, a capacitor connected between the base and the collector electrodes of the transistor, a first resistor connected in series with a switching means which are jointly connected between a first source of control potential and the base electrode, a second source of control potential connected via a second resistor to the base electrode, and means for developing an output signal at the collector electrode. By this circuit, when the switch means is closed, the rate of onset or attack of the output signal is linear, and when the switching means is opened, the rate of decay of the output signal is exponential.

m CONTROL POTENTIAL f sauna: ATTACK) a p75 76 rm '1 C22 A I CONTROL DIRECT L 1 POTENTIAL CONTROL t"" SOURCE (DECAY) R251: 1 l l l 1 TONE 1 GENERATOR C7 02 l J 1 32a 1 l l l l L PATENTED FEBTSIS?! v 3564.107

SHEET 2 UP 3 4 "K" K FIG. 2

CONTROL POTENTIAL 4K R45 SOURCE wvw (PERCUSSION) l I CONTROL POTENTIAL SOURCE (ArrAcK) TRI TO GA TE 0R MODULA TORS S CONTROL 74v 1 POTENTIAL SOURCE (DECAY) CONTROL POTENTIAL SOURCE ,L (DECAY) CONTROL POTENTIAL souRcE (A r TA CK 70 GA TE 0/? MODULA TORS L CONTROL POTENTIAL SOURCE fiPERcuss/oN) FIG- 3 INVENTOR. ALFRED B. FREEMAN i KEYING DRIVE CIRCUITS PARTICULARLY FOR ELECTRONIC MUSICAL INSTRUMENTS This is a continuation of application Ser. No. 599,931, filed I Dec. 7, 1966, now abandoned.

. this invention relates to keying drive circuits for driving keying circuits with proper voltage functions to produce desiredtone signal envelope patterns in response to playing key operation.

The direct switching of tone signals during the playing of an electronic musical instrument, such as an electronic organ,

produces abrupt starting and stopping of the tone signals. A

, desired musical effect, particularly in the case of electronic organs, is the lingering of the note being played after the playing key, has been released. Direct keying causes this effect to be lost, because the note ceases to sound immediately upon release of the playing key. Direct keying also results in undesirable transients such as key clicks when a playing key is depressed. 1 In order to avoid direct keying, it is known to employ various .types of resistance-capacitance networks to produce gradually rising and falling voltages in response to playing key operation to drive a diode or other type of gate of modulator .(keying circuit). The exponential decay of voltage when a capacitor discharges through a resistor conforms substantially ,to the envelope shape for the natural decay of a tone signal from an acoustical musical instrument. However, the exponential increase in voltage during charging of a capacitor does not correspond to the envelope shape for the natural attack of a tone signal from an acoustical musical instrument. A clipping circuit can be used to restrict the part of the charging voltage function which is used for gating or modulating (keying) to a substantially linear portion. While this provides an improved attack envelope more closely simulating the attack envelope of a tone signal from an acoustical musical instrument, it is difficult to adjust over the range of charging rates .which would be required for the production of different instrumental effects from electronic musical instrumentssuch as electronic organs. Since the attack and decay portions of the envelope of a tone signal are very important to instrumental quality and to musical effects, the ability to obtain economically a variety of attack and decay rates and shapes thus is an important feature of electronic musical instruments which attempt to provide sounds simulating that of many different acoustical musical instruments.

In accordance with this invention, there are provided economic keying drive circuits capable of producing a-variety of voltage functions corresponding to the different envelope patterns characteristic of different acoustical musical instruments and capable of being adjusted easily to produce voltage functions having different rates and shapes. Such keying drive circuits can be provided with controls for selecting the different instrumental effects which are such that the player only has to operate a single control to obtain both the proper spectrum or tonal waveform and the proper envelope.

In brief, a circuit embodying this invention comprises a transistor having base, emitter and collector electrodes, a source of potential, a capacitor, a resistor, a source of variable control potential, switching means, and means for developing an output signal at the collector electrode. The emitter electrode is connected to the source of potential, while the capacitor is connected between the base and collector electrodes. The switching means and the resistor are connected in series between the source of variable control potential and the base electrode so as to enable the control signal to be supplied to the base electrode from the source of variable control potential when the switching means isclosed;

This invention will become more apparent from the following detailed description, taken in conjunction with the appended drawings, in which:

FIG. I is a circuit diagram showing a keying drive circuit embodying this invention and some of the components of an electronic musical instrument with which it may operate;

FIG. 2 isa circuit diagram illustrating another embodiment of this invention which has a percussion-response capability;

FIG. 3 is a-circuit diagram showingyet another embodiment of this invention, which, like the circuit of FIG. 2, has a percussion-response capability; and

FIG. 4 is a circuit diagram showing one form of a control arrangement which may be used with any of the keying drive circuits shown in FIGS. 1 to 3.

Referring first to FIG. 1, there is shown a control potential source 14 for attack which provides a voltage output of adjustable magnitude that determines the attack rate of the signal produced by the keying drive circuit, as will become more apparent hereinafter. The output terminal of source I4. is connected to a bus bar 13. Also provided is a control potential source 17 fordecay which provides a voltage output of ad justable magnitude that determines the decay rate of the signal produced by the keying drive circuit, as will become more apto resistor R15 but associated with other switches. Bus bar 16 also is connected to the output terminal of control potential source (decay) 17. Switch 12a also is connected via a resistor R18 to the base electrode of a PNP transistor TRl. The emitter electrode of transistor TRI is connected'to a source of positive potential (8+), while the collector electrode of the transistor is connected through a resistor R21 to ground. A capacitor C22 is connected between the collector and base electrodes of transistor TRI, while a diode D1 is connected between 8+ and the base electrode of the transistor.

Two'tone generators 32a and 32b each producing a different tone signal are provided. Connected to the output terminal of each tone generator is a keying circuit that, in the embodiment shown, simply consists of diodes D2 and D3, the cathode of each diode being connected to the output terminal of its respective tone generator. The anodes of diodes D2 and D3 are connected via resistors R26a and R26b to bus bars 27a 27b respectively. Bus bars 27a and 27!; are connected to the base electrodes of transistors TR2a and TR2b respectively which serve as mixing amplifiers. The emitter electrodes of transistors TR2a and TR2 b are connected to ground, while the collector electrodes of these transistors are connected via 'resistor R291: and R29b respectively to a source of negative potential (13-). Negative feedback'and bias resistors R304 and Y R30b are connected between the base and collector electrodes of transistors TR2a and TR2b respectively. The output signals from these transistors are supplied to the stops and output system 31 of the organ. System 31 may include tone filters, control switches, amplifiers andsound transducers such as are normally found in electronic musical instruments.

The output signal from transistor TRl is supplied to a bus bar 24 connected to the collector electrode of transistor TRl. This signal is supplied to the two keying circuits via resistors R25a and R25b, the former being connected between bus bar 24 and the anode of diode D2, and the latter being connected between bus bar 24 and the anode of diode D3. If additional tone signals are to be keyed from transistor TRI, other tone generators and keying circuits may be connected as shown in FIG. 1, the output signals from these circuits being supplied to other bus bars like 270 and 27b via resistors like R260 and R26b, and the input signals from transistor TR] being supplied transistor TR1 will be at cutoff when switch 12a is open. The output voltage of source (attack) 14 should be variable from a voltage which is slightly less positive than 8+ to a voltage which is considerably more negative than B+.

When switch 12a is closed connecting the base electrode of transistor TR1 to bus bar 13 and the output terminal of source 14, base current will flow, and the amount of current passing through resistor R18 will be determined by the output voltage of source 14, since the base of transistor TR1 will remain at substantially the same voltage. In this connection, if transistor TR1 and diode D1 both are of germanium types, the change in voltage at the base electrode of transistor TR1 will be less than about 0.5 volts. The current passing through resistor R18 divides and will flow into the base of transistor TR1 and also sill charge capacitor C22. Since one plate of capacitor C22 is connected to the collector electrode of transistor TR1 the voltage at this collector electrode only can move towards more posivtive as capacitor C22 charges. Thus the voltage at point A (FIG. 1) will change linearly at a rate that is proportional to the current passing through resistor R18, except for the change caused by the base electrode drawing more current as transistor TR1 has to draw more current. The departure from linearity caused by base current will be small except for minimum attach rates when the output of source 14 just will be sufficient to pull transistor TR1 into saturation in the steady state condition with switch 12a closed.

When switch 12a is opened, the voltage at point A will fall exponentially, since capacitor C22 has to supply base current to transistor TR1, and thus discharges at a progressively slower rate as the base current decreases to cutoff. Additional discharge current will be provided via resistors R15 and R18 from source 17 when the output voltage thereof is more positive than B+. This additional discharge current may be used to increase the rate of discharge of capacitor C22 by an amount determined by the output voltage of source 17. The discharge rate of capacitor C22 thus can be varies from that which is determined by its size and the base current drive which is required for transistor TR1 to a considerably faster rate.

Capacitor C22 should be selected so as to provide the desired maximum decay time in combination with the gain of and load on transistor TR1. Source 17 should be such as to be capable of delivering a maximum output voltage sufficient to produce the shortest discharge time required. Diode D1 is provided so as to prevent the base electrode of transistor TR1 from being pulled very far positive even though the output voltage of source 17 may be considerably more positive than B+.

Tone generators 32a and 32b produce tone signals that swing from substantially ground potential to a positive value. When transistor TR1 is cut off (switch 12a open), diodes D2 and D3 will be reverse biased and will block the tone signals from the tone generators, preventing these signals from reaching mixing amplifier TR2, However, when transistor TR1 conducts and its collector electrode is pulled positive, diodes D2 and D3 will be forward biased, and the tone signals generated by tone generators 32a and 32b will be applied through resistors 26a and 26b respectively to bus bar 27 and thence to the base electrode of transistor TRZ. The magnitude of the signals supplied to bus bar 27 under these circumstances will be proportional to the voltage at point A, so that the envelopes of the tone signals will correspond to the waveform at point A. In other words, the tone signals will be amplitude modulated by the waveform at point A.

The particular tone generators, diode keying circuits, mixing amplifier and output system shown in FIG. 1 are by way of illustration only. It will be appreciated that the voltage waveform at the collector electrode of transistor TR1 could be used to drive a variety of other types of gating or modulating circuits to produce signal envelopes with adjustable rates of envelope attack of substantially linear shape and with adjustable decay rates.

If desired, and as shown in FIG. 1, the magnitudes of the output voltages from sources 14 and 17 can be controlled by the player by means of a control 34 mechanically coupled to sources 14 and 17. Alternatively, sources 14 and 17 may be controlled directly by the stops, which also may be mechanically coupled to sources 14 and 17. This latter arrangement allows a single control to select both tone quality and the attack and decay rates to obtain a desired instrumental quality.

Referring now to FIG. 2, a control potential source for percussion effects has been added, as has a switch 120, resistors R45 and R42 and a capacitor C44. The position of switch 12c as well as the position of switch 12a is controlled by playing key 110. Resistor R42 is connected between resistors R15 and R18 while resistor R45 is connected between switch 120 and B+. Capacitor C44 is connected between the common terminal of switch 12c and resistor R45 and the common terminal of resistors R42 and R18. As in the case of FIG. 1, the cathode of diode D1 is connected to B+. It will be noted that control potential source (percussion) 100 has its output terminal connected to a bus bar 40 which is contacted by switch 120 when closed.

If the variable output voltage ofsource 100 is at a value substantially the same as B+, the operation of the circuit shown in FIG. 2 when switches 12a and 12c are closed will be the same as the operation (if the circuit of FIG. 1, except that capacitor C44 will have the effect of delaying the buildup of voltage across resistor R18 and so the buildup of charging current to capacitor C22. This will give an increasing rate of charge with time, which williapproximate a rising exponential envelope signal shape, This attack envelope shape is a good approximation of the attack envelope shape of some acoustical musical instruments. The time constant of capacitor C44 in combina tion with resistors R42 and R18 should be made slightly longer than the longest attack for which this rising exponential attack shape effect is desired.

When the voltage output of source 100 is more negative than B+, the closing of switch 120 will abruptly change the voltage at the junction of capacitor C44 and resistor R45, so that the voltage on the other plate of capacitor C44 must change instantaneously by the same amount, This will start capacitor C22 charging through resistor R18. If the output voltage of source 14 is substantially the same as B+, capacitor C44 will charge to the saturation base voltage of transistor TR1 and then further to the cutoff voltage thereof as capacitor C22 discharges. The size of capacitor C44 should be selected such that, in combination with resistor R18, capacitor C44 will discharge only slightly during the time required for capacitor C22 to charge and transistor TR1 to reach saturation on a fast attack. This is necessary to avoid an appreciable falling off of the attack rate, such as would occur if the current through resistor R18 fell appreciably during this time. Capacitor C44 and resistor R43 also must be selected such that capacitor C44 soon will discharge completely and allow the decay to start with the discharge of capacitor C22. This is not difficult to achieve in practice, since the effect is useful only with fast attacks, so the delay before start of discharge can be short enough not to be perceptible.

The decay rate with switch 120 closed depends upon the output of source 14, which thus functions as the key down decay control. If the output voltage of source 14 is such that transistor TR1 is held saturated, there will be no decay until key is released. If the voltage output of source 14 is decreased from this value, the decay will move toward smaller and smaller levels as transistor TR1 is held partially on by different amounts. When the voltage output of source 14 is such that transistor TR1 reaches cutoff, the decay will be completely percussion. A more positive output voltage from source 14 than this value would further shorten the key down decay. Thus it is possible to have a variety of attach rates by adjusting the voltage output of source 100 and a variety of percussion decay rates by adjusting the voltage output of source 14 as well as combinations of percussion and sustain components.

When switch is opened by releasing key 110, capacitor C44 will discharge through resistors'R18 and R45 and diode D1 and will pull current from capacitor C22 and the base electrode of transistor TRI. This will produce a relatively fast key up decay, such as occurs when the key of a piano is released and the strings damped. The rate of key up decay can be increased further by adjusting the output voltage of source 17.

. An alternative arrangement of the circuits of FIGS. 1 and 2 isa circuit which will operate transistor TRl in the saturated state for the off condition and in the cutoff state for the on condition. Such a circuit is shown inFlG. 3 and it will be apparent how removal of certain of the components from the keying drive circuit of FIG. 1 would render the circuit of FIG. l 'operable in a similar manner. In thekeying drive circuit of FIG. 3, transistor TRl is replaced by a transistor TR3 of the NPN type which has its emitter electrode connected to ground and its collector electrode connected via a resistor R51, which replaces resistor R21, to the output terminal of control potential source (decay) 17. As in the case of FIG. 1, an output voltage is developed at the collector electrode of transistor TR3 which may be used to drive circuitsof the same type as those shown in FIG. 1. Of course, PNP transistors could be used in FIG. 3 or NPN transistors used in FIG. 3 or NPN transistors used in FIGS. 1 and 2 if different polarities of drive were required for the gating circuits or modulators. The similarity of the remainder of the circuit to the circuit shown in FIG. 2

will be apparent. It will be seen that resistor R has been eliminated, however, and that the output terminal of source 17 is connected via a resistor R52 to the base electrode of transistor TR3, this resistor being sized so as to just hold transistor TR3 in saturation for the load imposed by resistor R51 and the gates or modulators connected to the collector electrode of transistor TR3. It will be appreciated that it may be necessary to select resistor R52 to match the gain of each particular transistor TR3 and its particular load. In addition, a diode D3 is connected between the collector electrode of transistor TR3 and B+, so that the output signal from the keying drive circuit will not exceed the maximum signal required this can be varied over a considerable range without changing the exponential shape, because the drive current required also increases proportionately as more current required also increases proportionately as more current is drawn through resistor R51. Small departures will occur if the gain of transistor TR3 changes with current and resistor R51 is not the total load. An adjustable decay rate thus is provided while the envelope shape remains substantially in the form of the desired exponential shape rather than cutting off sharply for shorter decays, as in the case of prior art keying drive circuits and those shown in FIGS. 1 and 2.

When transistor TR3 is in the saturated condition, closing of switches 12a and 120 will provide a drive for cutoff and for the charging of capacitor C22. The current through resistor R18 must be greater, of course, than that passing through resistor R52, and the additional current is used to charge capacitor C22 to allow transistor TR3 to be pulled out of saturation, just as it allowed transistor TRl to be pulled into saturation in the circuits of FIGS. 1 and 2.

While sources l4, l7 and 100 can be controlled directly by the player to obtain the desired envelope shapes and rates, this may require'an appreciable time and may be awkward in the playing of instruments in which frequency change of instrumental quality is desired during the playing of a composition. In electronic organs and the like, changes in the tonal filters and footages used are controlled by operation of stop tabs to obtain different instrumental characteristics. Since attack and decay characteristics also are very important in determining instrumental quality, it would be advantageous to have the same stop-tab controls select the proper attack and decay shapes. An arrangement for accomplishing this result is shown in FIG. 4, to which reference is now made. In the circuit of FIG. 4, a mixer 58 receives its input signal from gating circuits or modulators and may be, for example, a circuit such as the circuit employing transistor TRZ in FIG. I. A stop tab 59 is mechanically coupled to switches 60a and 60b. Closing of switch 60a connects the output terminal of mixer 58 through a tone filter 61 whose output is connected to a mixer and output system 62. A stop tab 63 likewise is mechanically coupled to switches 64a, 64b and 64c, and closing of switch 64a likewise connects the output terminal of mixer 58 to a tone filter 65 whose output terminal is connected to mixer and output system62. The action of switches 60a and 64a, tone filters 61 and 65, and mixer and output system 62 is similar to that in many electronic organs and the like where similar elements select the timbre of sounds reaching the output.

Switch 60b connects ,a line 66 to ground when stop tab 59 is operated. Line 66 is connected to a common terminal of resistors R67 and R68 which constitute a part of a voltage divider that includes also resistors R69 and R70, these resistors being connected in series with each other between B- and ground. The commonterminal of resistors R67 and R69 is connected to control potential source (attack) 14. Grounding of line 66 by closing switch 60b changes the input to source 14 and consequently the attack rate obtained. Switch 64b likewise grounds line 72 when closed. Line 72 is connected to the junction of resistors R68 and R70 and, when grounded, changes the attack rate of source 14 by a different amount than the change efiected by grounding line 66. Likewise switch 640 grounds line 73 when closed. Line 73 is connected to the common terminal of resistors R74 and R75 that constitute part of a voltage divider which also includes a resistor R76, these three resistors being connected between 8+ and ground. The common terminal of resistors R76 and R75 is connected to control potential source (decay) 17, so .that operation of switch 640 will alter the decay rate. If both stop tabs 59 and 63 are operated, the decay rate will be that for stop tab 63 and the attack rate will be that for stop tab 59. It will be appreciated that various other combinations of controls and arrangements may be used to obtain similar results and that these arrangements may be expanded to include more controls and control of source (percussion) as well.

While preferred embodiments of this invention have been disclosed herein, those skilled in the art will appreciate that changes and modifications may be made therein without departing from the spirit and scope of this invention as defined in the appended claims.

Iclaim:

1. Apparatus for producing a variable subaudio direct current potential comprising: a transistor having base, emitter and collector electrodes; a source of direct current potential, said emitter electrode being connected to said source of potential; a first capacitor connected between said base and collector electrodes; a first source of variable subaudio direct current control potential; first switching means; a first resistor, said first switching means and said first resistor being connected in series between said first source of variable control and said base electrode, whereby a control current may be supplied to said base electrode from said first source of variable control potential when said switching means is closed; and means for developing a direct current output potential at said collector electrode.

2. Apparatus according to claim 1 including a second variable direct current control potential connected to said base electrode.

3. Apparatus according to claim 1 wherein said first source of variable subaudio direct current control potential produces a control potential varying from slightly less positive than the potential of said source of direct current potential to more negative than the potential of said source of direct current potential.

4. Apparatus according to claim 2 including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased.

5. Apparatus according to claim 1 including a second source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said second source of variable control potential and said emitter electrode, said second capacitor being connected between said second source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode.

6. Apparatus according to claim 1 including a second source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said second source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode via said first resistor.

7. Apparatus according to claim 2 including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; at third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode.

8. Apparatus according to claim 2 including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode via said first resistor.

9. The invention according to claim 1 including a tone generator for producing a tone signal to be amplitude modulated by said output signal; a modulator for amplitude modulating said tone signal with said output potential; and means for supplying said output potential and said tone signal to said modulator.

10. Apparatus according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and further including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased.

. 11. Apparatus according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and further including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor, a second resistor; said second switching means and said second resistor being connected in seriesbetween said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode.

12. Apparatus according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of p otential and further including a diode connected between said 'base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode via said first resistor.

13. The invention according to claim 2 wherein said first source of variable control potential produces a control p0tential varying from slightly less positive than the potential of said source of potential to more negative than said source of potentialfand wherein said second source of variable control potential produces a control potential from about the same potential as said source of potential to more positive than the potential of said source of potential and also including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone signal with said output potential; and means for supplying said output potential and said tone signal to said modulator.

14. The invention according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and also including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for. amplitude modulating said tone signal with said output potential; means for supplying said output potential and said tone signal to said modulator; and a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased.

15. The invention according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and also including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone signal with said output potential; means for supplying said output potential and said tone signal to said modulator; a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode.

16. The invention according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and also including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone signal with said output potential; means for supplying said output potential and said tone signal to said modulator; a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode via said first resistor. 1

17. Apparatus according to claim 1 including a second source of variable direct current control potential connected to said collector electrode.

18. Apparatus according to claim 1 including a second resistor, and a second source of variable direct current control potential connected to said collector electrode via said second resistor.

19. Apparatus for producing a variable subaudio direct current potential comprising: a transistor having base, emitter and collector electrodes, said emitter electrode having means for being connected to a positive direct current bias; a capacitor connected between said base and collector electrodes; first and second sources of subaudio direct current control potential; switching means; first and second resistors, said first resistor connected in series with said switching means between said first source of direct current control potential and said base electrode for supplying a direct current to said base electrode when said switching means is closed; means connecting said second source of direct current control potential via said second resistor and said base electrode; and means for developing a direct current output potential at said collector electrode.

20. Apparatus according to claim 19 including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone signal with said output potential; and means for supplying said output potential and said tone signal to said modulator.

21. Apparatus according to claim 19 in which said first source of direct current control potential is variable and is limited to produce a selected direct current control potential falling in the range from a potential slightly less positive than said positive bias to a potential still more negative than said positive bias. V

22. Apparatus according to claim 19 in which said sources of direct current control potential are variable.

23. In an electronic musical instrument having means for keying tone signals responsive in amplitude to a keying drive voltage, apparatus for producing the keying drive voltage, comprising:

a. a transistor having base, emitter and collector electrodes, said emitter electrode having means for being connected to a source of potential;

b. a capacitor connected between said base and collector electrodes;

c. first and second sources of control potential;

d. switching means;

e. first and second resistors, said first resistor connected in series with said switching means between said first source of control potential and said base electrode for supplying a control current to said base electrode when said switching means is closed;

f. means connecting said second source of control potential via said second resistor and said base electrode; and

g. means connecting the collector electrode to said keying means.

24. In an electronic musical instrument having means for keying tone signals responsive in amplitude to a keying drive voltage, apparatus for producing the keying drive voltage, comprising:

a. a transistor having base, emitter and collector electrodes,

said emitter electrode having means for being connected to a source of potential;

b. a first capacitor connected between said base and collector electrodes c. a first source of variable control potential;

d. first switching means;

e. a first resistor and said first switching means being connected in series between said first source of variable control potentiahand said base electrode so that a control current is supplied to said base electrode from said first source of variable control potential in response to closing of said switching means; and

f. means connecting the collector electrode to said keying means.

25. An electronic musical instrument according to claim 24 comprising:

a. a further control potential source;

b. a second switching means; and

c. a second capacitor having a series combination connection with said second switching means, said series combination having connections to the base of said transistor and to said further control potential source.

26. An electronic musical instrument according to claim 23,

comprising:

a. a'third control potential source;

b. a second switching means; and

c. a second capacitor having a series combination connection with said second switching means, said series combination having connections to the base of said transistor and to said third control potential source.

27. In an electronic musical instrument having means for keying tone signals responsive in amplitude to a keying drive voltage, apparatus for producing a percussive keying drive voltage, comprising:

a. a transistor having base, emitter and collector electrodes, said emitter electrode having means for being connected to a source of potential;

b. a first capacitor connected between said base and collector electrodes;

c. a first source of control potential;

d. switching means;

e. a second capacitor having a series combination connection with said switching means,'said series combination having a connection to said base electrode and a connection to said first source of control potential; and

f. means connecting the collector electrode to said keying means.

28. An electronic musical instrument according to claim 27 including a resistor connected in said series combination with said second capacitor and said switching means.

29. An electronic musical instrument according to claim 27 including:

a. a second source of control potential; and

b. a resistor connected between said second source and said series combination of said second capacitor and said switching means.

Patent No.

Inventor(s) Dated February 16, 1971 Alfred B. Freeman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col.

Col. 6,

Col. 7,

1. 11-13, delete "source of variable control pots and said emitter electrode, said second capacitor being connected 1 said second".

Signed and sealed this 19th day of October 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Attesting Officer ROBERT GOTTSCHALK Acting Commissioner of Pai 

1. Apparatus for producing a variable subaudio direct current potential comprising: a transistor having base, emitter and collector electrodes; a source of direct current potential, said emitter electrode being connected to said source of potential; a first capacitor connected between said base and collector electrodes; a first source of variable subaudio direct current control potential; first switching means; a first resistor, said first switching means and said first resistor being connected in series between said first source of variable control and said base electrode, whereby a control current may be supplied to said base electrode from said first source of variable control potential when said switching means is closed; and means for developing a direct current output potential at said collector electrode.
 2. Apparatus according to claim 1 including a second variable direct current control potential connected to said base electrode.
 3. Apparatus according to claim 1 wherein said first source of variable subaudio direct current control potential produces a control potential varying from slightly less positive than the potential of said source of direct current potential to more negative than the potential of said source of direct current potential.
 4. Apparatus according to claim 2 including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased.
 5. Apparatus according to claim 1 including a second source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said second source of variable control potential and said emitter electrode, said second capacitor being connected between said second source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode.
 6. Apparatus according to claim 1 including a second source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said second source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode via said first resistor.
 7. Apparatus according to claim 2 including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode.
 8. Apparatus according to claim 2 including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode via said first resistor.
 9. The invention according to claim 1 including a tone generator for producing a tone signal to be amplitude modulated by said output signal; a modulator for amplitude modulating said tone signal with said output potential; and means for supplying said output potential and said tone signal to said modulator.
 10. Apparatus according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and further including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased.
 11. Apparatus according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential proDuces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and further including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor, a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode.
 12. Apparatus according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and further including a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode via said first resistor.
 13. The invention according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential from about the same potential as said source of potential to more positive than the potential of said source of potential and also including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone signal with said output potential; and means for supplying said output potential and said tone signal to said modulator.
 14. The invention according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and also including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone signal with said output potential; means for supplying said output potential and said tone signal to said modulator; and a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased.
 15. The invention according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and also including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone sigNal with said output potential; means for supplying said output potential and said tone signal to said modulator; a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode.
 16. The invention according to claim 2 wherein said first source of variable control potential produces a control potential varying from slightly less positive than the potential of said source of potential to more negative than said source of potential, and wherein said second source of variable control potential produces a control potential variable from about the same potential as said source of potential to more positive than the potential of said source of potential and also including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone signal with said output potential; means for supplying said output potential and said tone signal to said modulator; a diode connected between said base and emitter electrodes with the polarity required to prevent said base electrode from being excessively back biased; a third source of variable control potential; second switching means; a second capacitor; a second resistor; said second switching means and said second resistor being connected in series between said third source of variable control potential and said emitter electrode, said second capacitor being connected between said second switching means and said base electrode via said first resistor.
 17. Apparatus according to claim 1 including a second source of variable direct current control potential connected to said collector electrode.
 18. Apparatus according to claim 1 including a second resistor, and a second source of variable direct current control potential connected to said collector electrode via said second resistor.
 19. Apparatus for producing a variable subaudio direct current potential comprising: a transistor having base, emitter and collector electrodes, said emitter electrode having means for being connected to a positive direct current bias; a capacitor connected between said base and collector electrodes; first and second sources of subaudio direct current control potential; switching means; first and second resistors, said first resistor connected in series with said switching means between said first source of direct current control potential and said base electrode for supplying a direct current to said base electrode when said switching means is closed; means connecting said second source of direct current control potential via said second resistor and said base electrode; and means for developing a direct current output potential at said collector electrode.
 20. Apparatus according to claim 19 including a tone generator for producing a tone signal to be amplitude modulated by said output potential; a modulator for amplitude modulating said tone signal with said output potential; and means for supplying said output potential and said tone signal to said modulator.
 21. Apparatus according to claim 19 in which said first source of direct current control potential is variable and is limited to produce a selected direct current control potential falling in the range from a potential slightly less positive than said positive bias to a potential still more negative than said positive bias.
 22. Apparatus according to claim 19 in which said sources of direct current control potential are variable.
 23. In an electronic musical instrument having means for keying tone signals responsive in amplitude to a keying drive voltage, apparatuS for producing the keying drive voltage, comprising: a. a transistor having base, emitter and collector electrodes, said emitter electrode having means for being connected to a source of potential; b. a capacitor connected between said base and collector electrodes; c. first and second sources of control potential; d. switching means; e. first and second resistors, said first resistor connected in series with said switching means between said first source of control potential and said base electrode for supplying a control current to said base electrode when said switching means is closed; f. means connecting said second source of control potential via said second resistor and said base electrode; and g. means connecting the collector electrode to said keying means.
 24. In an electronic musical instrument having means for keying tone signals responsive in amplitude to a keying drive voltage, apparatus for producing the keying drive voltage, comprising: a. a transistor having base, emitter and collector electrodes, said emitter electrode having means for being connected to a source of potential; b. a first capacitor connected between said base and collector electrodes; c. a first source of variable control potential; d. first switching means; e. a first resistor and said first switching means being connected in series between said first source of variable control potential and said base electrode so that a control current is supplied to said base electrode from said first source of variable control potential in response to closing of said switching means; and f. means connecting the collector electrode to said keying means.
 25. An electronic musical instrument according to claim 24 comprising: a. a further control potential source; b. a second switching means; and c. a second capacitor having a series combination connection with said second switching means, said series combination having connections to the base of said transistor and to said further control potential source.
 26. An electronic musical instrument according to claim 23, comprising: a. a third control potential source; b. a second switching means; and c. a second capacitor having a series combination connection with said second switching means, said series combination having connections to the base of said transistor and to said third control potential source.
 27. In an electronic musical instrument having means for keying tone signals responsive in amplitude to a keying drive voltage, apparatus for producing a percussive keying drive voltage, comprising: a. a transistor having base, emitter and collector electrodes, said emitter electrode having means for being connected to a source of potential; b. a first capacitor connected between said base and collector electrodes; c. a first source of control potential; d. switching means; e. a second capacitor having a series combination connection with said switching means, said series combination having a connection to said base electrode and a connection to said first source of control potential; and f. means connecting the collector electrode to said keying means.
 28. An electronic musical instrument according to claim 27 including a resistor connected in said series combination with said second capacitor and said switching means.
 29. An electronic musical instrument according to claim 27 including: a. a second source of control potential; and b. a resistor connected between said second source and said series combination of said second capacitor and said switching means. 